The invention relates to improvements in apparatus for transporting webs or strips of paper, plastic sheet stock or metallic foils, and more particularly to improvements in apparatus for transporting webs of sheet material which can be converted into a series of blanks for use in conjunction with the making or wrapping of packets of cigarettes or other types of smokers' products. Still more particularly, the invention relates to improvements in methods of and in apparatus for transporting webs of wrapping material in such a way that a web which is to be converted into a series of successive blanks is advanced along a predetermined path by at least two conveying units including (a) an upstream conveying unit and (b) a downstream conveying unit employing two pneumatic suction band conveyors which engage and entrain the two marginal portions of the web.
The making of satisfactory packets for arrays of plain or filter cigarettes in a cigarette packing machine renders it necessary to advance the wrapping or packing material (normally in the form of webs or strips which are to be subdivided into a series of successive blanks of a prescribed size and shape) with a very high degree of accuracy, predictability and reproducibility. This ensures that a packing machine can turn out short or long series of identical eye-pleasing packets regardless of the number of envelopes or layers in a packet and irrespective of whether the rod-shaped articles are confined in so-called soft packs, in so-called hinged lid packs or any other selected forms of receptacles. Predictable conveying of continuous and subdivided webs or strips of wrapping material is particularly important during certain specific stages of a cigarette wrapping or packet making operation, as well as in connection with the conveying of certain types of highly sensitive wrapping material such as webs of metallic foil (normally or often aluminum foil) which are or which can be draped directly around arrays of plain or filter cigarettes prior to confinement of such arrays in cardboard boxes serving for the making of hinged lid packs.
It is equally important to properly manipulate certain very thin and highly sensitive webs or strips of plastic material, such as transparent plastic material which is utilized for the making of outermost envelopes or layers; such layers are often provided with so-called tear strips to facilitate rapid and convenient removal of the outer envelopes when a purchaser desires to gain access to the contents of a cigarette packet. The outer envelopes are often made of polypropylene foil which is designed to preserve the aroma and/or to prevent changes in the moisture content of cigarettes or other rod-shaped smokers' products.
Commonly owned U.S. Pat. No. 4,999,667 to Gottfried Hoffmann discloses an apparatus wherein a web of wrapping material to be converted into blanks for the making of cigarette packets is advanced lengthwise from a roll or another suitable source along a predetermined path by a first suction-operated conveying unit having two spaced-apart foraminous conveyor bands each serving to attract and advance one of the two marginal portions of the web, and thereupon by a second suction-operated conveying unit which is a mirror image of the first conveying unit and also employs two spaced-apart foraminous conveyor bands each attracting and entraining one of the two marginal portions of the web downstream of the first conveying unit. A cross cutter is located between the two conveying units to subdivide the web into a series of successive blanks which are engaged by successive packets and are draped around the respective packets while the packets are caused to cross the path of the blanks between the foraminous bands of the second conveying unit. Successive packets, with the respective blanks partially draped around them, are propelled into successive chambers at the periphery of an indexible turret in the packing machine, and the conversion of blanks into outer envelopes of the packets is completed while the packets are confined in the chambers of the turret.
It has been ascertained that, though the patented apparatus operates quite satisfactorily, the material of the web is likely to develop fold lines, wrinkles, creases and/or to undergo other undesirable deformation during transport from the first to the second conveying unit, especially if the web is made of thin aluminum foil or of thin plastic (such as polyethylene) foil. The development of wrinkles, creases or the like detracts from the appearance as well as from the aroma and/or moisture preserving quality of the respective envelopes.
The disclosure of the U.S. Pat. No. 4,999,667 to Hoffmann is incorporated herein by reference.
A different web transporting apparatus is disclosed in European patent No. 0 545 265 A2. This apparatus employs a first suction-operated web conveying unit with three parallel foraminous band conveyors disposed between two knives which serve to cut into the marginal portions of the advancing web. The first conveying unit is followed by a second conveying unit employing a succession of pairs of suction-operated pincers or tongs which engage the front corners of the web and are mounted on two driven endless belts flanking the sides of the path for the web. The purpose of the tongs is to pull the web to a station where successive portions of the web are propelled or otherwise forced into the range of deforming instrumentalities. The just described apparatus transports the web from a lower level to a higher level, and each of a series of successively formed blanks (which are obtained in response to repeated severing of the web by a cross cutter) is suspended on the respective pair of suction-operated tongs. A drawback of this proposal is that the retention of successive blanks in optimum positions is least likely in the region of the second conveying unit, i.e., exactly in the region where the blanks are to be guided with utmost precision in order to ensure optimal draping around arrays of cigarettes or around partially finished cigarette packets.